1. Field of the Invention
The present invention is generally related to an amplifier circuit for use with a pressure transducer and, more particularly, to a circuit which exhibits temperature independence and a high degree of accuracy at very low magnitudes of pressure throughout a wide range of temperatures. The amplifier circuit of the present invention provides an amplifier that can be manufactured less expensively because of its total elimination of the need for thermistors for temperature compensation.
2. Description of the Prior Art
Pressure transducers of many types are well known to those skilled in the art. One particular type of pressure transducer utilizes piezoresistive components in a solid state circuit to measure pressure. The piezoresistors are disposed in a semiconductor material which is shaped in the form of a diaphragm. As the diaphragm is distorted, as a result of pressure against the diaphragm, the conductive characteristics of the piezoresistors change. By monitoring the voltage drop across the resistors, the change in resistivity of these devices can be used to determine the magnitude of the pressure on the diaphragm. For example, U.S. Pat. No. 4,656,454, which issued to Rosenberger on Apr. 7, 1987, describes a piezoresistive pressure transducer with an elastomeric seal. It discloses a low cost piezoresistive pressure transducer that utilizes premolded elastomeric seals. The pressure transducer is particularly adapted for automatic assembly. It provides a means for holding a piezoresistive stress sensitive element in the form of a diaphragm of semiconductor material between a pair of premolded elastomeric seals in a thermoplastic housing. Electrical connections with external circuitry are made with strain relief jumpers which connect conductive regions on the element outside the seals to conductors which pass through the housing wall. FIG. 3 of the patent provides a representative illustration of a solid state piezoresitive sensor device of the general type which is useable in association with the present invention.
U.S. Pat. application Ser. No. 07/630-687, which is titled "RIBBED AND BOSSED PRESSURE TRANSDUCER" and assigned to the assignee of the present application issued to Johnson et al. as U.S. Pat. No. 5,156,052 on Oct. 20, 1992 and discloses a particular physical configuration of a silicon diaphragm used in association with a plurality of piezoresistive devices to improve the sensitivity and accuracy of a pressure transducer of this type. Although the present invention can be utilized with many different types of pressure transducers, the ribbed and bossed pressure transducer described in this patent application is particularly adaptable for use in association with the present invention.
Pressure transducers which utilize piezoresistive components typically experience a nonlinearity in response to temperature fluxuations. As a result, the output voltage signal provided by the pressure transducer is not constant as a function of pressure on the diaphragm if the transducer is subjected to changes in temperature. This problem requires that the transducer be provided with some means for compensating for temperature changes. A typical, and probably the most common, technique for temperature compensation in pressure transducers is to utilize thermistors in the compensation circuit. A thermistor, which is a thermally sensitive solid state semiconducting device that is usually made by sintering mixtures of the oxide powders of various metals, typically requires a ceramic substrate on which the thermistor is deposited by the use of thick film technology. A temperature compensation circuit which utilizes thermistors therefore necessitates the inclusion of a ceramic substrate, or equivalent component, for use in support of the thermistors. Present technology finds difficulty in attempting to produce thermistors in a semiconductor substrate through the use of most known semiconductor manufacturing techniques.
When used in conjunction with piezoresistive components that are implanted or diffused in semiconductor material, the thermistors commonly exhibit temperature coefficients that vary widely, requiring trimming resistors and special temperature testing and trim algorithms during manufacture. Therefore, beside increasing the overall cost of the pressure transducer, the use of thermistors exacerbates the difficulties encountered in providing a temperature compensation circuit for a pressure transducer of this type. In addition, the requirement of a ceramic substrate to support the thermistors of the circuit virtually precludes the further miniaturization of the pressure transducer circuit by preventing the pressure transducer from being entirely manufactured on a single silicon chip.
It would therefore be highly desirable if a pressure transducer can be manufactured with temperature compensation capability without the use of thermistors in the amplification or compensation circuits of the pressure transducer. This would eliminate the need for the thermistors which ar difficult to accurately calibrate and which require the use of a ceramic substrate for their support. The elimination of the need for thermistors would also eliminate the need for the use of thick film technology in association with pressure transducer circuits and would eventually permit the entire pressure transducer, along with its amplification and linearization circuitry, to be manufactured on a minimum number of semiconductor substrates.